A conventional vehicle environmental control system typically comprises a mechanically-driven compressor for an air conditioning system, as well as an alternator to charge a storage battery, augment power supplied by the battery, and power electrical fans to circulate air for the air conditioning system. In such arrangements a prime mover engine of the vehicle must be operating for the air conditioning system to function. This type of environmental control system has a number of shortcomings when installed in vehicles such as buses and ambulances. Firstly, mounting kits for the compressors are relatively complex and often vary between manufacturers. In addition, the compressors themselves typically require regular maintenance and often have reliability issues. Furthermore, the alternators are relatively unreliable, expensive and inefficient. High idle from an associated prime mover engine of the vehicle may also be required to operate the compressor in a manner sufficient to achieve adequate cooling, thereby resulting in relatively poor fuel economy for the vehicle.
In contrast, electrically-driven air conditioning systems have benefits in terms of installation flexibility and simplified belt drives between the air conditioning system and the vehicle's engine. As used herein, the term “air conditioning” is used in the broadest sense and is not meant to preclude air heating and filtrating functions affecting a vehicle's environmental air supply. Electrically-driven air conditioning systems also have further benefits over engine-driven systems, such as improved reliability and durability, substantially reduced failure modes, and reduced warranty costs. A further advantage of electrically-driven air conditioning systems over engine-driven systems is increased capacity and coefficient of performance (COP) which is described in Equation 1:
                    COP        =                  Q          W                                    Equation        ⁢                                  ⁢        1            where Q is the average heat removed through an evaporator of the air conditioning system (evaporator cooling capacity) divided by W, the average work input to the compressor. It is important to note that increased capacity and COP can be achieved without changing the evaporator or condenser of the electrically-driven air conditioning system.
Electrically driven air conditioning systems also offer the opportunity to have full cooling capacity at engine idle, eliminating the need for “high idle,” improving overall system efficiency and life expectancy of the cooling system as well as the overall system. This is particularly true if the air conditioning systems are combined with additional direct-current (DC) electrical power and with a heating component, such as electrical heating powered from one of the electrical power buses in the system or a fuel-fired heating system fueled from a fuel tank of the vehicle. The benefits of this arrangement include “instant-on” heat while the engine is cold and reduced thermal load on the engine, which can be a problem for vehicle emissions. In addition, engine heating can be provided.
Many vocational vehicles have additional electrical loads that are required to be operational on the vehicle for the vehicle to be effective for its intended purpose. In addition to DC power, portable alternating-current (“AC”) electrical power is often a necessary resource for many specialized vehicles. For example, emergency vehicles often require AC power to operate medical equipment carried onboard the vehicle. AC power is likewise used in utility, construction vehicles and fire trucks to operate various kinds of tools and equipment. AC power may also be used to operate wheelchair lifts on buses. Another common use for vehicle-based AC power is in long-haul transport tractor-trailer trucks equipped with a sleeper compartment wherein AC power may be used to operate convenience accessories such as electric razors, televisions and microwaves. AC power is also the most convenient form of power for driving a motor and air conditioning compressors, in particular, are most readily available integrated with AC motors. These motors are well-established as highly reliable and relatively low-cost components. Refrigerated vehicles can also benefit greatly from this AC power technology.
Ambulances have relatively high-capacity needs for air conditioning on a relatively small vehicle. Because they provide cooling for both the cab and the rear patient compartment, they require augmentation to the basic vehicle platform on which they are typically built. There is also a requirement for increased electrical power capacity to operate lights, a power inverter such as a DC-AC inverter to run medical devices, and so on. This requirement conflicts with the design of the base vehicle, which is built so as to satisfy a wider market, particularly with today's drive for improved fuel economy.
Increasing efficiency, fuel economy, and environmental needs are also increasingly requiring capability to use the vehicle's accessories without operating the vehicle's engine. This often requires “export power” from the electrical system to run electrified accessories or loads such as a refrigerated body, leading to a need for greater energy storage and management of that resource. This is particularly the case for vehicles wherein the prime mover engine typically idles for considerable periods of time, such as shuttle buses, delivery trucks, and ambulances. Other benefits from reducing or eliminating idling are reduced maintenance costs and improved reliability, which is often quantified in terms of “mean time between failures” (MTBF).
Static inverters are commonly used to generate portable AC electrical power output from a DC power source input. Such inverters are relatively lightweight and have few, if any, moving parts to wear out. In addition, inverters do not require a fueled engine (such as the vehicle's prime mover) to produce power, are quiet, and do not produce fumes. Inverters are also more efficient than comparable power sources, such as motor-driven generators. However, inverters suffer from a limitation in that their output power, measured in volt-amps (“VA”) or watts, may be constrained under some conditions. For example, an inverter that derives its input power from a vehicle's alternator system may not be able to deliver the full amount of electrical power demanded by a load when the vehicle is at idle, since the power delivery capacity of an alternator varies directly with the vehicle's engine speed.
Aside from the need to generate AC power there is a desire on the part of many vehicle manufacturers to increase the “electrification” of vehicles, i.e., reducing the number of accessories that depend directly on the fueled-engine as a mechanical prime mover. Example accessories include power steering pumps, hydraulic drives, engine cooling fans, air conditioning compressors, oil and coolant pumps, and air compressors. Advantages of accessory electrification include a reduction in engine loading, which facilitates greater engine performance, increased flexibility in locating and mounting the accessories in the vehicle (particularly as available space in the engine compartment becomes more scarce), reduced fuel consumption, operation of pumps at an optimal speed independent of engine speed, more efficient accessory operation made possible by optimizing the location and wiring of the accessories, simplified plumbing, reduced environmental impact, and reduced vehicle emissions corresponding to reduced engine loading and fuel consumption.
Some vehicles may have several battery power supplies. For example, a vehicle may have a first battery system or an ultra-capacitor for operating a starter to “crank,” or start, the prime mover engine, and a second battery system for powering accessories. The discharge and load characteristics can vary considerably between the cranking and accessory batteries. For example, cranking batteries and ultra-capacitors are intended to provide high current for a relatively short period of time to start the engine, while the accessory batteries are used to provide a smaller amount of current to the vehicle's accessories for a relatively long period of time. Accordingly, the accessories may be powered by a “deep cycle” battery configured to have a long cycle life (e.g., from substantially full charge to substantially discharged) rather than high current delivery. The types of batteries used for cranking and for powering accessories may also differ. For example, a cranking battery may use flooded lead-acid batteries while the accessory battery may use deep-cycle absorbed glass mat (“AGM”) batteries. Each type of battery can have differing charge requirements. Likewise, ultra-capacitors may be employed in vehicle electrical systems in lieu of conventional batteries to serve as the cranking power source.
There is a need for a way to control battery charging to an amount appropriate for each battery in a vehicle electrical system having multiple batteries. There is a further need for a way to control and route power between multiple power supplies and distribution buses in a vehicle in order to supply and augment the buses and charge the batteries as needed or desired. There is also a need to manage the discharge and recharge of the batteries to minimize engine run time, and optimize battery life.
There are also systems in the field that use electrically driven air conditioners powered from battery supplies, the batteries being recharged by the standard vehicle prime mover engine's alternator. There are numerous problems associated with such systems. Firstly, the conventional vehicle electrical system voltage is lower than optimum for a typical air conditioning compressor, as the compressor requires a significant amount of power to drive it. In addition, the alternator is challenged to recharge the batteries while also supplying the air conditioning power. More typically, the air conditioning cannot be used while the vehicle is being driven. Furthermore, the cooling available is often limited in order to limit the current drawn by the compressor. Electrically-driven air conditioning systems also have a relatively limited time duration of operation due to factors such as limitations relating to battery size. A further drawback is that the batteries for these systems are often relatively large and heavy because of the intended “overnight” operation capability of the air conditioning systems. The systems are also relatively inefficient, as they use alternators designed for low cost with old technology rather than performance. Additionally, the systems place a relatively heavy burden on the vehicle electrical system, consequently reducing the system's reliability.
As can be seen from the foregoing discussion, there is a need for a power management and environmental control system that addresses the drawbacks and limitations of current systems.